In season HS Training:
As you have hopefully realized by now, I believe in progression throughout the training cycle. There are no abrupt changes. Even when adding in or increasing the emphasis on a particular aspect, there is a gradual transition. This same principle applies when transitioning from the summer base phase into the in season competition phase of training.
At this point in the training, many programs make a distinct change going from almost all easy runs to starting to hammer out the intervals. In my mind, this is a recipe for injury, overtraining, or a premature peak. We want to gradually and progressively bring the body to peak shape in as predictable a way as we can. If we throw the kitchen sink at the athletes by introducing interval training all at once, the predictability of when an athlete is going to run his best and how he is going to progress is thrown right out the window.
To keep things as straight forward as possible, I’ll stick with the same idea of showing the progression of various workouts over the course of the season.

Competition Period:

Specific Work:
The easiest workouts to show progression are the specific workouts. If we look at where we left off at the end of summer with our specific workout and then come up with a final specific workout to do, the progression is very natural and easy. Towards the end of the summer training, the athletes were doing either 200s or 60sec pickups at specific or near specific paces (3k-5k). These workouts were really just spices of running at race pace. Now it is time to formally develop specific endurance.
This is done best by lengthening the rest distance or reducing the recovery. But wait, before we start the progression from the spices of race pace work to specific endurance development, we need to decide on where we are going with the workout. That means, deciding the last hard specific workout before the race. That workout provides a destination. If you have the starting point and the finish point, then coming up with the in between is relatively simple. In designing a final workout, keep in mind that you want it to be a challenging workout that is highly specific to the race. That means relatively long intervals at the pace they want to run with relatively short rest in between.

Brad Hudson, in his book, suggests working towards 5x1km with 60-90sec rest at 5k pace. It’s a simple progression going from 200s or 400s at race pace to 1,000s at race pace. Increase the rep distance progressively towards 1,000m, decrease the rest from 2:30 (for example) down to 1 minute, and increase the total distance covered from 2 miles (for example) to 5k.
I tend to prefer using one of two slightly different progressions that are a little more complex and have worked well for me in the past. You can use them independently or combine both progressions.

The first is similar to the first method but with a slight change. I start with short intervals, but instead of starting with long rest and decreasing it, I start with very short rest in between reps but split the session into sets so that it is feasible. In addition, the total volume of the session matches the total volume of the final workout from the beginning. An example of an introductory workout for an athlete is 3 sets of 4x400m with 30-40sec rest at 5k pace and 5 minutes in between sets. Just like in the other example, you gradually increase the distance of the reps. Instead of systematically decreasing the rest in between the reps, you slowly decrease the number of sets (and thus the total rest) working towards a repetition workout with just 1 set. Below is an example of this progression.

The other method that I like to use is the alternation workout. In this workout you combine high end aerobic running with race pace work. For High School 5k runners, I like to use 4 miles of total work. You alternate running at goal pace with a medium to high end aerobic pace. Like in the other specific work, you gradually increase the amount of race pace work done. These alternation workouts provide a different kind of stimulus than just traditional race pace work and is a great way to create specific endurance. Below is an example of a progression of alternation workouts:

I usually stop at 800m with HS kids but occasionally will go up to 900 or 1,000m for advance runners.

These are just a couple examples of how to effectively create specific endurance. Combining some of these workouts is a great way to diversify the training a bit and makes sure that every runner gets a training stimulus that is effective. Since you are working with a large group of diverse athletes in HS, it’s good to switch up the program to make sure that each runner gets some sort of training stimulus he responds to. Also, you get to see what works best for each runner for future reference. Below is an example of combining two methods of creating specific endurance

All of these methods in creating specific endurance work well. What one you use depends on the type of athlete you have (remember FT vs. ST), and what you are trying to accomplish. For example, using the alternation approach will probably be more suited to a ST athlete because the ‘rests’ are kept at a decent pace. A FT runner, might not be able to accomplish the workout because he won’t recover as quickly on the rests. On the other hand, using some of the alternation progression might help that FT athlete improve his ability to deal with lactate while running at a steady pace. So he is working on his weakness. Consider this example and the athletes themselves when choosing your preferred progression.

how would you apply your philosophy in regards to coaching a decent sized high school team of 25 kids? i know thing will have to be individualized in regards you don't want a 15 minte 5k runner to be doing what a 20 minute 5k runner is doing and vice versa. just for you informtion i'm the only coach; so the pointers can be specific to the situation even though you don;t know each individual runner.